[System] Add [MonoTODO] on DefaultTraceListener.AssertUiEnabled.

See the mono-list thread for fun/gory details:

http://lists.ximian.com/pipermail/mono-list/2014-March/050732.html
Add new public API function mono_set_crash_chaining.

mono_set_crash_chaining tells mono to chain crash signals instead of just
running the next one.

This allows us to get the crash info from both mono and the native system.
[xbuild] Fix building with 2.0 toolset

BXC18794 - Error executing task Csc with 2.0 toolset since Mono 3.4.0
[jit] Fix some of the check for support of atomic intrinsics on arm.